The purpose of this randomized controlled trial is to evaluate the effectiveness of an empirically supported psychosocial treatment, Acceptance and Commitment Therapy, in facilitating improved quality of life, benefit-finding, and cortisol rhythm in breast cancer patients in an outpatient clinical oncology setting. Previous research indicates that breast cancer patients may demonstrate disrupted diurnal cortisol rhythms compared to healthy individuals, and that these disrupted rhythms may be related to recurrence and earlier mortality in some patients. Interestingly, improvements in cortisol regulation in previous intervention studies for cancer patients have not necessarily been related to decreased distress. Rather, improvements in post-traumatic growth, benefit-finding, and meaningfulness have also accounted for improved neuroendocrine and immunological changes. Traditional breast cancer groups, however, may not adequately address these areas because existing interventions often target the reduction of distress as the primary vehicle to improve psychosocial, quality of life, and biophysical outcomes. Acceptance and Commitment Therapy (ACT) is an empirically-supported, mindfulness-based psychological treatment that has been shown to enhance meaningful behavior change thorough increasing emotional acceptance of difficult psychological experiences such as distress, without the goal of changing or eliminating them. The current study seeks to determine the preliminary effect of an 8-week ACT group in increasing positive life changes and corresponding increase in salivary cortisol slope in 40 distressed breast cancer patients, who will be randomly assigned to ACT or a wait list control group. The hypotheses for the present study include: 1) that patients receiving ACT will demonstrate improvements in Quality of Life (QoL), Benefit-finding (BF), and health behavior compared to control group participants;2) ACT participants will demonstrate improvements in mean cortisol levels and cortisol reactivity compared to control group participants, and 3) these changes will be the result of increased mindful acceptance of cancer-related distress and meaningful behavior changes, rather than a reduction in distress. Psychosocial outcomes for these patients will include differences in QoL, BF, and health behaviors and will be assess pre-treatment, post-treatment, and at 3-month follow-up. Stress reactivity will be measured by assessing diurnal cortisol patterns over three days before treatment and at 3-month follow-up. Analyses will be conducted to determine the relative contribution of distress reduction versus acceptance on psychosocial and cortisol outcomes. This study provides an opportunity to perform a pilot evaluation of an empirically-supported psychosocial treatment for breast cancer patients, while examining whether patients in this distressing situation can achieve psychological and diurnal cortisol rhythm changes through engaging in meaningful behaviors, rather than distress reduction. PUBLIC HEALTH RELEVANCE: Being diagnosed with and treated for breast cancer is often a distressing experience. Recent research has demonstrated that benefits in quality of life, stress hormone reactivity, and immune system functioning can be achieved by helping women with breast cancer find benefit and growth through the cancer process, even though the experience is stressful. This study examines the impact of a treatment designed to target meaningful life changes on quality of life and stress responses in the body.